Resist and Post-Etch Removal Uses& Moist Ozone Gas Phase Chemistry
Staff -- Semiconductor International, 10/1/1998
There
is increasing demand for improved wafer cleaning processes that follow the plasma
etching of ever smaller submicron structures. The success of low resistance
interconnections in high-density ULSI devices is dependent on via hole cleanliness.
The stripping and cleaning technique used must remove all types of sidewall
polymer residue that remain after dry etching. Many methods use wet processing
with solvent strippers. These are effective, but costly, and give rise to health,
safety and environmental problems.
Researchers at IMEC (Leuven, Belgium) have devised a moist ozone (O3) gas process for the removal of photoresist and organic post-etch residues from the surfaces of silicon wafers. The improved performance over traditional processes is due to an enhanced availability of reactive O3 near the wafer surface. This process is environmentally friendly, provides an alternative to using harmful solvent strippers and is said to eliminate effectively need for various process steps in IC production that are based on sulfuric acid.
This IMEC process involves exposing wafers to a moist O3 gas phase environment. An OH radical scavenger may be added to enhance chemical process efficiency. The oxidation efficiency is raised further by maximizing the O3 concentration at elevated temperatures, while the wet boundary layer on the wafer surface is controlled. A thin layer of condensation is produced on the wafer surface by the moist gas, so a continuous and concentrated supply of O3 is maintained. The process can remove completely a layer of resist in less than 10 min.
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Fig. 1. This SEM image of 0.6 µm over-etched via structures was taken after conventional oxygen dry strip etching. |
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| Fig. 2. This SEM image of 0.6 µm over-etched via structures was taken after moist gas phase O3 processing with the addition of acetic acid. |
When a control wafer was dry stripped for 45 min in a conventional oxygen (O2) plasma, typical post-etch polymer residues were clearly visible (Fig. 1). Similarly, after 10 min treatment in a control experiment in which wafers were immersed in a heated liquid just above an O3 diffuser, so that O3 and O2 bubbles contacted the surface, not all of the resist layer was removed. However, a 10 min treatment using the moist O3 gas phase system rapidly stripped both deep and aluminum over-etched via structures of both resist and post-etch polymer residues (Fig. 2). This moist O3 ambient process depends on both chemical and physical enhancement of the oxidation efficiency through use of OH scavengers and a moist O3 gas phase ambient.
"The effectiveness of our ozone gas process is immediately apparent when compared
with the 45 minute cycle required by existing ozone techniques, such as O2
dry strip," said Gilbert Declerck, vice president of the Advanced Semiconductor
Division at IMEC. 
